1. Field of The Invention
The present invention relates to a vehicle automatic transmission with means for automatically controlling the gear shifts thereof, and particularly relates to a hydraulic control unit for controlling the operation of the automatic transmission.
The present invention also relates to a hydraulic control unit for an automatic transmission with means to appropriately engage a frictional engaging element for engine braking.
2. Description of The Prior Arts
Automatic Transmissions for use on automobiles and other vehicles have a plurality of power transmitting gear trains each of which has a specific rear ratio. These transmitting gear trains are selectively switched into operation to effectuate a gear shift by controlling engagement or disengagement of frictional engaging means such as clutches and brakes. The engagement of the frictional engaging means is hydraulically controlled by a hydraulic control unit. The hydraulic control unit often comprises a hydraulic regulator valve for producing a line pressure, and an operating pressure control means for producing an operating pressure supplied to the frictional engaging means by regulating the line pressure. The operating pressure control means is provided to reduce a shift-shock which is caused when engaging the frictional engaging means. Specifically, a duty-ratio on-off controlled solenoid valve is often used for the operating pressure control means. An apparatus to reduce a shift-shock during a shift is disclosed in the Japanese Patent Publication No. 61-136055.
In the automatic transmission, a specified frictional engaging means which is engaged at a specified gear position (such as at a reverse gear position) is sometimes required to have a fairly large engaging force because of the gear ratio between the power source (the engine) and the specified frictional engaging means. To obtain such a large engaging force, a high operating pressure is often needed.
Therefore some hydraulic control units have a line pressure switching means to control the regulator valve so as to produce a high line pressure and a low line pressure. The high line pressure is used at the specified gear position (such as at the reverse gear position) and the low line pressure is used at the other gear positions (such as at the forward gear positions).
However, the engagement of the specified frictional engaging means under the high line pressure may cause a jerky shift which is not preferable. It is desirable to control the operating pressure control means so as to supply a low operating pressure during engagement (from the start of engagement until the completion of engagement), and then to supply a high operating pressure after completion of engagement to firmly maintain the engaging state of the specified frictional engaging means.
However, if the high line pressure is supplied to the operating pressure control means to produce the low operating pressure, it is difficult to produce it from the high line pressure. For example, relationships between duty-ratios (%) and operating pressures (kg/mm.sup.2) in the duty-ratio on-off controlled solenoid valve (of normal open type) are shown in FIG. 6. As shown by a line "B" in the figure, if the high line pressure (P2) is used, the duty ratios required to produce the low operating pressures to engage the specified frictional engaging means smoothly (the operating pressures within the shaded range in the figure) must be controlled within a very narrow range. Since the duty ratios are controlled in correspondence with the electric voltage supplied to the solenoid, it is very difficult to control the duty ratio within the narrow range.
In an automatic transmission, as described above, shift controls are carried out by selecting a gear train (a power transmitting path) from a plurality of gear trains which are included in the transmission. The gear train is selected by engaging one or two frictional engaging means such as clutches and brakes. A specified gear position corresponding to a specified gear train, for example 1ST gear position, often includes a one-way clutch or one-way brake. The one-way clutch or one-way brake allows only one-way transmission of power, i.e. it allows a power transmission from an engine to wheels, but it does not allow a power transmission from the wheels to the engine. Accordingly, when the 1ST gear position is selected with a one-way clutch being operated, no engine brake can be effectuated during deceleration running.
However, engine brake is sometimes required during deceleration running at 1ST gear position to effectively slow down the vehicle speed. In order to make the engine brake available, a specified frictional engaging means (an engine-brake engaging means) is engaged to directly connect an input member with an output member of the one-way clutch, allowing the power transmission from the wheels to the engine.
Meanwhile, a torque converter which is included in an automatic transmission often comprises a lock-up means. The lock-up means as well as the engine brake engaging means are usually operationally controlled by a common operational pressure.
One example of a prior art hydraulic circuit to control operations of the engine brake engaging means and the lock-up means is shown in FIG.10.
The hydraulic circuit comprises an engine brake frictional engaging element B2, a switching valve 45 by which an operating hydraulic pressure is selectively supplied through a shuttle valve 56 to the engine brake engaging means B2 and a lock-up clutch (L/C) of a torque converter (not shown), a solenoid valve SE (an operating pressure control means) which controls the operating pressure supplied to the switching valve 45, and a manual valve 25 which controls the supply of a line pressure P1 (a base pressure) to the solenoid valve SE and which is shifted to a position corresponding to a shift range selected based on the driver's operation of a shift lever. It further comprises a solenoid valve SD which controls a supply of a pilot pressure Pp to the switching valve 45 to control the operation of the switching valve. In U.S. Pat. No. 4,936,166, a hydraulic circuit in which an engine brake engaging means and a lock-up clutch control means are controlled by a common pressure supplied from one solenoid valve is disclosed.
In the above constructed hydraulic circuit, when the manual valve 25 is shifted to a position corresponding to "1-range", oil under the line pressure P1 exerted from a pump (not shown) and regulated by a regulator valve is supplied to the solenoid valve SE. Then, when the solenoid valve SE is opened, the oil under the line pressure P1 is supplied to the switching valve 45.
Since the solenoid valve SD is closed and the pilot pressure is not applied on the switching valve 45 at 1-range, a spool 46 of the switching valve 45 is shifted to the left biased by the line pressure P1 applied on the right end thereof, whereby the operating pressure is supplied to the engine brake engaging means B2. Accordingly, the solenoid valve SE can control the operation of the engine brake engaging means B2 by a supply control of the line pressure thereto to control the engine braking.
When the manual valve 25 is shifted to a D-range, the operating pressure is supplied to the switching valve 45 through the opened solenoid valve SE. Since the solenoid valve SD is opened at D-range, the pilot pressure Pp is supplied to the left end of the switching valve 45 to move the spool 46 to the right. As a result, the operating pressure is supplied to the lock-up means no control the operation thereof. Therefore at D-range, when 1ST gear position is selected, the engine brake engaging means B2 is not engaged and no engine brake is available.
However, even if the manual valve is shifted to the D-range at which no engine braking is required, the switching valve may not shift to the lock-up control side because of, for example, a malfunction of the solenoid valve SD or a sticking of the spool of the switching valve. When the switching valve is not shifted to the lock-up control side, the operating pressure exerted from the solenoid valve SE is not supplied to the lock-up control means but is supplied to the engine brake engaging means, which is not preferable for smooth running of the vehicle.